Dr. Frieden, a 48-year-old infectious disease specialist, has cut a high and sometimes contentious profile in his seven years as New York's top health official under Mayor Michael R. Bloomberg. He led the crusade to ban smoking in restaurants and bars, pushed to make H.I.V. testing a routine part of medical exams, and defended a program that passes out more than 35 million condoms a year.

At the C.D.C., he will inherit a host of immediate and long-term problems, including a looming decision about whether and how to produce a swine flu vaccine. Health experts say the agency must resolve serious morale and organizational issues even as the administration struggles to overhaul the nation's health care system.

"I think the administration selected Tom Frieden because he can take public health to a new place," said Jeffrey Levi, executive director of Trust for America's Health, a nonprofit public health advocacy organization. "He's a transformational leader."